Modern, large horizontal axis wind turbine-generators generally include two or more variable pitch blades mounted on a rotor which drives a synchronous generator through a gear box, the gear box serving to step up the rotational speed of the main turbine shaft to the speed required for synchronous operation of the generator. In such wind turbine-generators, it is generally desirable to control blade pitch in response to wind conditions and turbine output requirements. A suitable blade pitch control system for large wind turbine-generators is disclosed in U.S. Pat. No. 4,193,005 to Kos et al. This control system is a closed-loop system which provides a blade pitch angle reference signal to a blade pitch change actuation system based on such parameters as wind conditions, desired turbine-generator operating conditions and actual turbine-generator operating conditions. The system of the Kos et al patent includes four discrete controllers: a first controlling rotor acceleration during start up, a second controlling rotor deceleration during shutdown, a third controlling rotor speed when the synchronous generator is off-line and a fourth controlling power or torque when the generator is on-line. The controllers provide a time derivative pitch angle reference signal as an input signal to an integrator. The output of the integrator is the blade pitch angle reference signal noted hereinabove. The integrator includes maximum and minimum blade angle stops. The maximum stop corresponds to a blade pitch angle of 90.degree. (full blade feathering) while the minimum blade angle stop is variable, being a function of rotor speed and measured wind speed.
As explained in the Kos et al patent, the blade pitch angle reference signal set by the controller is limited to a minimum value (the minimum integrator stop) under wind velocity conditions between cut-in velocity (the minimum wind velocity at which the wind turbine-generator is capable of producing useful power), and rated velocity (the minimum wind velocity at which the wind turbine-generator may produce rated power). It is crucial to the operation of the wind turbine that in this range of wind velocities, the pitch angle of the blades be set to such a minimum value for capture of the greatest possible amount of energy from the wind stream. At wind velocities greater than those within this range, more than enough wind energy is available for the generation of rated power and hence, energy is "spilled"0 from the blades as the wind turbine operates. However, within this range, the blades must be precisely set at minimum pitch angles wherein no significant amount of energy is spilled from the blades.
In the Kos et al control system, values of minimum pitch angle (.beta..sub.min) as a function of velocity ratio .lambda. (a product of a constant, the blade diameter and frequency of blade rotation divided by wind velocity) are stored in a function generator or memory 204. It will be appreciated then, that in order to provide an accurate .beta..sub.min signal, function generator 204 must be input with accurate readings of wind velocity and rotor frequency of rotation. While the frequency of rotation of the rotor is easily and accurately measurable, an accurate determination of wind velocity is not so easily achieved. Heretofore, it has been the practice to measure wind velocity from a point sensor mounted at a location removed from the wind turbine rotor. It will be appreciated that such a sensor will only sense wind conditions at its immediate location and therefore, due to the presence of nonuniformities in wind velocity, may not accurately reflect the velocity conditions at the rotor. Moreover, even if the velocity sensor could accurately measure wind velocity conditions at the rotor, such a measurement would still be only indicative of wind velocity at the location of the sensor. For accuracy, the minimum blade pitch angle signal should be based on wind conditions integrated over the entire area of the rotor. Clearly, a point wind velocity sensor is not capable of such measurements. Under those low wind velocity conditions wherein minimum blade pitch angle controls the operation of the turbine, even a slight error in the determination of wind velocity can result in blade pitch angle settings wherein the amount of wind energy which the turbine is capable of capturing and, therefore, turbine output, are significantly jeopardized.